Sensor devices for determining the rotational speeds of wheels are known. Normally, the devices include an incremental encoder which is mechanically coupled to the rotating part, and a sensor which scans the encoder in a non-contact manner. Ferromagnetic toothed wheels or ferromagnetic discs are used as encoders. When sensors are used in wheel bearings, it is customary to employ magnetized structures, for example, an annular or circular arrangement of adjacent north and south poles, embedded in a mechanical carrier.
It is most common to use so-called "passive" sensors according to the reluctance principle. The sensors include a combination of a copper coil with a permanent magnet which is magnetically coupled to the encoder. The encoder modulates the magnetic coupling reluctance synchronously with movement. An alternating voltage, synchronous with movement, is induced in the copper coil in the way of a dynamo, and the frequency of the alternating voltage is evaluated as a measured quantity. The induced signal voltage fades away during standstill and will possibly adopt undesirably high values at high speeds.
To avoid such high voltages so-called "active" sensors, have been provided. Sensors of this type are described in German patent application No. 41 35 789, for example. The sensors are a combination of a magnetostatically sensitive element and a permanent magnet which is magnetically coupled to the encoder. In this arrangement also, the encoder modulates the magnetic coupling reluctance or, in the case of the magnetized encoder, the field direction, synchronously with movement. The sensor element responds to the variations of the flux density or to the movement of a field vector. Examples in the art of such basic elements are Hall probes or magneto-resistive permalloy structures. The signal amplitude of active sensors of this type is independent of the speed of the encoder. Active sensors require for operation an electric energy which is supplied by an electronic controller. Technical methods involving a three-conductor feed-in or a two-conductor feed-in are known in the art. The three-conductor version supplies a signal voltage to the input of the electronic controller, while the two-conductor version furnishes a signal current.
Active sensors of the current type, limit the sensor information available to them to one single parameter, the signal frequency. An internal amplifier trigger circuit generates a binary output signal with two constant amplitude values in the area of a defined, permissible air slot between the sensor and encoder, irrespective of the size of the air slot. The change in flanks of the amplitude values is evaluated, but the signal itself does not indicate the changed values. However, the signal discontinues abruptly when the permissible air slot size is exceeded because in this event the induced sinusoidal input signal falls short of a sensor-inherent hysteresis threshold, and the sensor stops responding.
A device for the adjustment of the air slot size, as disclosed in German patent application No. 40 33 064, ensures a proper mounting position of the sensor and maintains the permissible air slot size. The sensor is fitted in a sleeve-shaped mounting support and is urged by a compression spring with a flange against a corresponding flange of the mounting support in the direction of the encoder. The flange of the sensor includes a plurality of similar, axial teeth, while the flange of the mounting support includes two groups of axial indentations, the number of which corresponds to the number of teeth. When the sensor is installed, its teeth are placed in the group of the deep indentations, and the assembly of the mounting support is so that the sensor is initially in contact with the encoder. Subsequently, the teeth are locked in the group of the flat indentations so that a defined size of the air slot between the sensor and encoder is adjusted. The correct mounting position cannot be verified after the assembly is completed.
When an active sensor, due to misassembly, is inadvertently operated at the limits of its maximum permissible air slot, an appropriately great dynamic deformation of the kinematics between the sensor and encoder will possibly increase the air slot so that the permissible air slot size is exceeded and the output signal is interrupted.
Therefore, an object of the present invention is to render testing of the size of the air slot possible even after the assembly of the sensor.
It may be determined by change-over from a first to a second hysteresis threshold, which is higher than the first hysteresis threshold, whether the input signals are sufficient to overcome even a high hysteresis threshold. When the generation of output signals continues even after change-over to the second hysteresis threshold, it may be assumed that proper operation of the sensor is ensured even when taking into account dynamic axial deformations which increase the air slot.
Various provisions are possible as means of actuating a hysteresis change-over of this type. When the change-over is triggered by an increase in the operating voltage of the sensor, no special software arrangements in the controller or in a maintenance apparatus are required. This is advantageous especially when the testing of the air slot size shall be carried out only during maintenance of a vehicle.
If the electronic controller is preferred to perform testing of the air slot size automatically, it is advisable to actuate the sensor by serial bit configuration voltages because this way excessively increased supply voltages are not necessary, not even during testing cycles as mentioned above.
In a particularly simple manner, resetting to the first hysteresis threshold, which permits the sensor to sense the motion, is achieved by the sensor switching back automatically. This may occur after a defined period of time which is sufficient to test the air slot size, or instantaneously after interruption of a signal which causes change-over to the second hysteresis threshold. For safety reasons, resetting is provided also when the encoder reaches a predetermined speed.
The idea of the present invention will be explained in detail in the following, making reference to two two-part Figures of the accompanying drawing.